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					       Chapter 1
      Introduction




1.1
 1-1 DATA COMMUNICATIONS

Telecommunication means communication at a distance.
Data refers to information presented in whatever form.
Data communications exchange of data between two
devices via some form of transmission medium such as a
wire cable.


 TOPICS:

 Components
 Data Representation
 Data Flow

1.2
                      Data communications
 • Communicating devices must be part of a
   communication system made up of a combination
   of hardware (physical equipment) and software
   (program).
 • Characteristics of data communications system:
      –   Delivery (Deliver data to correct destination)
      –   Accuracy (Deliver data accurately)
      –   Timeliness (Deliver data in a timely manner)
      –   Jitter (variation in packet arrival time, uneven delay in
          the delivery of audio or video packets.)



1.3
          Components of data communications
 • Message:
      – The information (data) to be communicated. Popular forms of information
        include text, numbers, pictures, audio, and video.
 • Sender:
      – The device that sends the data message.
      – It can be a computer, workstation, telephone handset, video camera, and so
        on.
 • Receiver:
      – The device that receives the data message.
 • Transmission medium:
      – The physical path by which a message travels from sender to receiver.
      – e.g., twisted-pair wire, coaxial cable, fiber-optic cable, and radio waves.
 • Protocol:
      – A set of rules that govern data communications.
      – It represents an agreement between the communication devices.




1.4
      Five components of data communication




1.5
                        Data representation
 • Text
      – Text is represented as a bit pattern, a sequence of bits (0s or 1s).
      – The process of representing symbols is called coding.
      (ASCII)
 • Numbers-represented by bit patterns.
           - number is directly converted binary number.
 • Images
      – RGB (red, green, blue)
      – YCM (yellow, cyan, and magenta)
 • Audio
   - recording or broadcasting of sound or music.
 • Video
   -recording or broadcasting of a picture or movie.


1.6
                           Data flow
 • Communication between two devices can be
   simplex, half-duplex, or full-duplex (as shown in
   next slide).
 • Simplex
      – The communication is unidirectional, as on a one-way
        street.
 • Half-duplex
      – Each station can both transmit and receive, but not at
        the same time.
 • Full-duplex
      – Both stations can transmit and receive simultaneously.


1.7
  Figure 1.2 Data flow (simplex, half-duplex, and full-duplex)




1.8
 1-2 NETWORKS

A network is a set of devices (nodes) connected by
communication links.
A node can be a computer, printer, or any other device
capable of sending and/or receiving data generated by
other nodes on the network.




1.9
                       Distributed processing
 • Distributed processing, - task is divided among
   multiple computers.
 • Network Criteria
       – Performance(transit time, response time)
          • Throughput
          • Delay
       – Reliability
          • Measured by the frequency of failure
          • The time it takes a link to recover from a failure
          • The network robustness in a catastrophe
       – Security



1.10
                   Types of connections
 • A network is two or more devices connected through links.
 • Link: Physical communication pathway that transfers data from
   one device to another.




 Two types of connections:
               1. Point-to-Point
               2. Multipoint




1.11
1) Point-to-point:
-dedicated link between two devices.
-entire capacity of the channel is reserved for transmission.
Eg : Changing TV channels by infrared remote control.

2) Multipoint: (Multidrop)
-more than two specific devices share a single link.
-Channel capacity is shared (either spatially or temporally)
-devices uses link simultaneously-spatial
-devices take turns-temporal



1.12
       Types of connections




1.13
                     Physical Topology
 • Physical topology: in which a network is laid out physically.
 • Two or more devices connect to a link; two or more links
   form a topology.
 • The topology of a network is the geometric representation
   of the relationship of all the links and linking devices
   (usually called nodes) to one another.
 • Four basic topologies:
              »   Mesh
              »   Star
              »   Bus
              »   Ring




1.14
       Categories of topology




1.15
                   Mesh Topology
 • Dedicated point-to-point link.
 • ie. Link carries traffic only between two devices it
   connects.
 • MN:- n(n-1)/2 physical channels to link n devices.
        -every device must have n-1 i/o ports.




1.16
       A fully connected mesh topology (five devices)




1.17
                                 Mesh Topology

 • advantages
       – dedicated links (guarantees each link carries own data)
       – Robust (if one link is damaged, others work)
       – privacy or security (only intended recipient sees)
       – Ease fault identification and fault isolation
 •     disadvantages
       – The amount of cabling and the number of I/O ports required
       – Because every device connected with every other device, installation and
          reconnection are difficult.
       – wiring can be greater than the available space (in walls, ceilings, or floors)
       – h/w required to connect each link (I/O ports and cable) is expensive.

       Pbm: LD corporation consists of eight devices. Calculate total no. of cable
         links and no. of ports for each devices.


1.18
                       Star Topology
 • Each device has a dedicated point-to-point link only to a
   central controller, usually called a hub.
 • if one device wants to send data to another, it sends the
   data to the controller, which then relays data to the other
   connected device.

                                 A star topology connecting four stations




1.19
Advantages:
   •A star topology is less expensive than a mesh topology.
   •Easy to install and reconfigure
   •Far less cabling needs to be housed
   •Robustness: If one link fails, only that link is affected.


Disadvantage:
   •The dependency of the whole topology on one single point, the hub.
   If the hub goes down, the whole system is dead.




1.20
                       Bus Topology
 • A bus topology, on the other hand, is multipoint.
 • One long cable acts as a backbone to link all the devices in
   a network.
 • Nodes are connected to the bus cable by drop lines & taps.
 Dropline: Connection running b/w device & main cable.
 Tap : Connector either splices into the main cable.




                A bus topology connecting three stations

1.21
 Advantages:
    •Ease of installation
    •Less cabling
    •Redundancy is eliminated
 Disadvantages:
    •Difficult reconnection
    •fault isolation
    •Optimally efficient at installation, difficult to add new devices
    •Signal reflection at the taps can cause degradation in quality
    [A fault or break in the bus cable stops all transmission. The
    damaged area reflects signals back in the direction of origin,
    creating noise in both directions.]



1.22
                             Ring Topology
 • Each device is linked to only its immediate neighbors
   (either physically or logically).
 • Each device in ring has a repeater.
 • Advantages
   - Relatively easy to install and reconfigure
       – To add or delete a device requires changing only two connections.
       – Fault isolation is simplified.
       – A signal is circulating at all times. If one device does not receive a
         signal within a specified period, it can issue an alarm.
 • Disadvantages
       – Unidirectional traffic
       – In a simple ring, a break in the ring (such as a disable station) can
         disable the entire network.
       – It can be solved by using a dual ring or a switch capable of closing
         off the break.

1.23
              A ring topology connecting six stations

 • To connect n devices, n cable links are needed.
 • Eight device ring needs eight cable links.




1.24
  A hybrid topology: a star backbone with three bus networks




1.25
                            Protocols
 • An entity is anything capable of sending or
   receiving information.
 • A protocol defines what is communicated, how it is
   communicated, and when it is communicated.
 • Syntax
       – Refers to the structure or format of the data, meaning
         the order in which they are presented.
 • Semantics
       – Refers to the meaning of each section of bits.
 • Timing
       – When data should be sent and how fast they can be sent.

1.26

				
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posted:10/13/2012
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